In general, the use of plows to improve water drainage from sludge carried on an endless conveyor belt has been recognized to be advantageous. Thus, in U.S. Pat. No. 4,609,467, which is commonly assigned to the assignee of the present application, plows are disclosed which extend into the longitudinal path of movement of the belt and are rotatable in order to minimize hang-up of fibrous materials that may be undesirably retained on other, conventional plows. In that patent, the rotatable plows descend into the path of the sludge and thereby distribute the sludge evenly over the face of the belt while at the same time scraping or wiping the top surface of the belt to provide exposed filter media portions. Thereafter, existing free liquid flows by the force of gravity from the sludge to and through the liquid permeable belt on which the sludge is carried. The liquid is then collected and further treated or discharged in the course of the waste management program. The solids remaining in the sludge then pass to other apparatus for further treatment, such as a press section of a belt filter press.
Still another patent which constitutes prior art and is commonly assigned is U.S. Pat. No. 4,602,998, which describes a device for raising a plow assembly such that the plows may be lifted out of the path of the sludge on the belt in case that sludge contains fibrous materials that adhere to the plows and thereby inhibit their distribution and scraping function. Such an apparatus is definitely advantageous in order to remove fibrous or stringy material that may become entangled with the plows, as well as for general cleaning purposes. Indeed, the apparatus and function of the invention disclosed in the latter patent, i.e., facile raising of the plows out of the path of sludge, is advantageously utilized in conjunction with the presently disclosed apparatus.
The invention disclosed herein advances the concepts and structures contained in these two patents, as well as other patents directed to plows or flow breakers, such as U.S. Pat. No. 3,984,329, and greatly improves the capability of plows to facilitate the separation of liquid from a two-phase, solid-liquid mixture, or from a sludge that is being carried on a permeable filter belt. This is accomplished by creating a series of flow elevation zones with the creation of multi-directional eddy currents to improve mixing over essentially the entire surface of the permeable filter belt that is in contact with the sludge. The solids-liquid mixture is caused to form a turbulent layer covering all or a large part of the filter belt surface which, because of the characteristics of the turbulent layer, greatly increases the rate of filtration, or drainage, of the liquid from the sludge.
As the sludge initially is deposited upon the belt, an excess of free water begins to draw through the filter media carrying with it to the top surface of the media light floc and fines which tend to cover the top of the media and block or prevent maximum drainage. The first row of plows, when positioned properly, wipes the filter media and lifts and gently disturbs the light surface floc, thereby enhancing drainage and allowing the floc to build. As the sludge continues to travel toward the discharge end of the belt, additional liquid drains off and at each subsequent contact of plows of greater angle encounter increased resistance causing the sludge mass to build and at the same time releasing more liquid. Eventually the mass may become so great that upon engaging a plow of sufficiently large angle it is caused to roll over on itself laterally and longitudinally, thereby impinging and falling on oncoming sludge driving out additional liquid.
Where this phenomenon of the sludge causing mixing eddies to occur and even rolling back on itself in some instances, takes place at multiple locations along the length of the filter belt in the longitudinal direction, it has been determined that the solids content of sludge carried by the belt, without pressure being exerted on that sludge and without a vacuum being drawn from beneath the belt, can be increased up to nine-fold. This can be accomplished by the expedient of using plows according to the present invention and spacing and varying those plows in a manner so as to produce optimal results. As will be apparent to those of skill in the art, such spacing and number of plows utilized, either in a concentrated arrangement at the discharge end or a distributed arrangement along the length of the filter media, will be dependent, inter alia, on the type of sludge being dewatered, the rate of feed of the sludge onto the filter belt, and the solids content desired in the filter cake that is discharged from the filter media.
Accordingly, it is an important object of the present invention to provide an overall plow assembly system in which by a combination of the shape of the plows, the spacing of the plows, and variations in the plows, themselves, as those plows are situated in the path of the sludge carried on a filter media, a highly effective gravitational dewatering of the sludge is achieved.
It is another object of the present invention to provide a plow assembly in which the spacing of the plows, the shape of the plows, and other factors can easily be varied in order to achieve optimum dewatering. It is another object of the invention to provide individual plows of an optimal shape, themselves, so that by use of such plows flow elevation zones are created with the multi-directional eddy mixing that gives rise to maximum water release from the sludge.